Many conventional receivers employ a super heterodyne approach wherein the received frequency is converted to a relatively high intermediate frequency (IF) before demodulation. One of the major problems with wide frequency range coverage with this type of receiver is the large number of spurious responses which require extremely complex and complicated electronically tuneable filters. Further, because the intermediate frequency is relatively high, the layout of the intermediate frequency circuit is critical and requires shielding. Also, the local oscillator/synthesizer wideband shielding and decoupling becomes extremely complicated because RF submultiples can lie in the IF band.
Recently, receivers which convert the RF carrier to DC and the information modulated on the RF carrier to audio frequencies have been developed. These receivers have many advantages over the super heterodyne receivers, some of which are: no image signals or noise; spurious responses which are far from the center frequency, therefore, simplifying the preselector; the audio frequency IF allows a low power drain design with high dynamic range; and the IF filter is an inexpensive, noncritical low pass design.
The DC IF receivers also have several disadvantages. First, the signal will generally be several orders of magnitude lower than the amplifier and mixer DC offsets. Second, the receiver usually needs to be phased locked to the received carrier to properly demodulate the signal. The receiver IF filter is one of the components of the phase locked loop and limits the loop bandwidth. A narrow loop bandwidth eliminates using the phase locked loop as an FM demodulator. Even with a narrow phase locked loop, the unwanted DC offsets need to be we11 below the DC component due to the carrier. Special circuitry must be used to null this unwanted DC.
In most prior art transceivers, frequency modulation in the transmitter is done by modulating the local oscillator directly. Assuming that the local oscillator is a frequency synthesizer, several problems exist with FM modulation. First, the synthesizer contains a voltage controlled oscillator (VCO) the sensitivity of which usually changes with local oscillator frequency, resulting in a changing modulator gain. Second, the VCO is part of a phase locked loop which will track out the modulation if the loop bandwidth is too wide. This last problem leads to a narrow phase locked loop bandwidth, with accompanying slow acquisition and excessive phase noise, or it leads to a more complex modulation input network to compensate for the effects of the loop.